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Plant Cell Advance Online Publication
Published on July 1, 2005; 10.1105/tpc.105.032987

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Received March 26, 2005
Returned for revision May 19, 2005
Accepted May 25, 2005

RAC GTPases Mediate Auxin-Induced Formation of Auxin/Indole Acetic Acid-Containing Proteolytically Active Nuclear Protein Bodies in Tobacco and Arabidopsis Protoplasts

Li-zhen Tao 1, Alice Y. Cheung 2*, Candida Nibau 3, and Hen-ming Wu 4

1 Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003
2 Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003; Molecular and Cellular Biology Program, University of Massachusetts, Amherst, Massachusetts 01003; Plant Biology Graduate Program, University of Massachusetts, Amherst, Massachusetts 01003
3 Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003; Plant Biology Graduate Program, University of Massachusetts, Amherst, Massachusetts 01003
4 Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Massachusetts 01003; Molecular and Cellular Biology Program, University of Massachusetts, Amherst, Massachusetts 01003

* To whom correspondence should be addressed. E-mail: acheung{at}biochem.umass.edu.

Auxin signaling relies on ubiquitin ligase SCFTIR1-mediated 26S proteasome-dependent proteolysis of a large family of short-lived transcription regulators, auxin/indole acetic acid (Aux/IAA), resulting in the derepression of auxin-responsive genes. We have shown previously that a subset of Rac GTPases is activated by auxin, and they in turn stimulate auxin-responsive gene expression. We show here that increasing Rac signaling activity promotes Aux/IAA degradation, whereas downregulating that activity results in the reduction of auxin-accelerated Aux/IAA proteolysis. Observations reported here reveal a novel function for these Rac GTPases as regulators for ubiquitin/26S proteasome-mediated proteolysis and further consolidate their role in auxin signaling. Moreover, our study reveals a cellular process whereby auxin induces and Rac GTPases mediate the recruitment of nucleoplasmic Aux/IAAs into proteolytically active nuclear protein bodies, into which components of the SCFTIR1, COP9 signalosome, and 26S proteasome are also recruited.






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